1 //===-- WebAssemblyFrameLowering.cpp - WebAssembly Frame Lowering ----------==//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
11 /// \brief This file contains the WebAssembly implementation of
12 /// TargetFrameLowering class.
14 /// On WebAssembly, there aren't a lot of things to do here. There are no
15 /// callee-saved registers to save, and no spill slots.
17 /// The stack grows downward.
19 //===----------------------------------------------------------------------===//
21 #include "WebAssemblyFrameLowering.h"
22 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
23 #include "WebAssemblyInstrInfo.h"
24 #include "WebAssemblyMachineFunctionInfo.h"
25 #include "WebAssemblySubtarget.h"
26 #include "WebAssemblyTargetMachine.h"
27 #include "llvm/CodeGen/MachineFrameInfo.h"
28 #include "llvm/CodeGen/MachineFunction.h"
29 #include "llvm/CodeGen/MachineInstrBuilder.h"
30 #include "llvm/CodeGen/MachineModuleInfo.h"
31 #include "llvm/CodeGen/MachineRegisterInfo.h"
32 #include "llvm/Support/Debug.h"
35 #define DEBUG_TYPE "wasm-frame-info"
38 // TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions
40 /// We need a base pointer in the case of having items on the stack that
41 /// require stricter alignment than the stack pointer itself. Because we need
42 /// to shift the stack pointer by some unknown amount to force the alignment,
43 /// we need to record the value of the stack pointer on entry to the function.
44 bool WebAssemblyFrameLowering::hasBP(
45 const MachineFunction &MF) const {
47 MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo();
48 return RegInfo->needsStackRealignment(MF);
51 /// Return true if the specified function should have a dedicated frame pointer
53 bool WebAssemblyFrameLowering::hasFP(const MachineFunction &MF) const {
54 const MachineFrameInfo &MFI = MF.getFrameInfo();
56 // When we have var-sized objects, we move the stack pointer by an unknown
57 // amount, and need to emit a frame pointer to restore the stack to where we
58 // were on function entry.
59 // If we already need a base pointer, we use that to fix up the stack pointer.
60 // If there are no fixed-size objects, we would have no use of a frame
61 // pointer, and thus should not emit one.
62 bool HasFixedSizedObjects = MFI.getStackSize() > 0;
63 bool NeedsFixedReference = !hasBP(MF) || HasFixedSizedObjects;
65 return MFI.isFrameAddressTaken() ||
66 (MFI.hasVarSizedObjects() && NeedsFixedReference) ||
67 MFI.hasStackMap() || MFI.hasPatchPoint();
70 /// Under normal circumstances, when a frame pointer is not required, we reserve
71 /// argument space for call sites in the function immediately on entry to the
72 /// current function. This eliminates the need for add/sub sp brackets around
73 /// call sites. Returns true if the call frame is included as part of the stack
75 bool WebAssemblyFrameLowering::hasReservedCallFrame(
76 const MachineFunction &MF) const {
77 return !MF.getFrameInfo().hasVarSizedObjects();
81 /// Returns true if this function needs a local user-space stack pointer.
82 /// Unlike a machine stack pointer, the wasm user stack pointer is a global
83 /// variable, so it is loaded into a register in the prolog.
84 bool WebAssemblyFrameLowering::needsSP(const MachineFunction &MF,
85 const MachineFrameInfo &MFI) const {
86 return MFI.getStackSize() || MFI.adjustsStack() || hasFP(MF);
89 /// Returns true if the local user-space stack pointer needs to be written back
90 /// to memory by this function (this is not meaningful if needsSP is false). If
91 /// false, the stack red zone can be used and only a local SP is needed.
92 bool WebAssemblyFrameLowering::needsSPWriteback(
93 const MachineFunction &MF, const MachineFrameInfo &MFI) const {
94 assert(needsSP(MF, MFI));
95 return MFI.getStackSize() > RedZoneSize || MFI.hasCalls() ||
96 MF.getFunction()->hasFnAttribute(Attribute::NoRedZone);
99 static void writeSPToMemory(unsigned SrcReg, MachineFunction &MF,
100 MachineBasicBlock &MBB,
101 MachineBasicBlock::iterator &InsertAddr,
102 MachineBasicBlock::iterator &InsertStore,
103 const DebugLoc &DL) {
104 const char *ES = "__stack_pointer";
105 auto *SPSymbol = MF.createExternalSymbolName(ES);
106 MachineRegisterInfo &MRI = MF.getRegInfo();
107 const TargetRegisterClass *PtrRC =
108 MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
109 unsigned Zero = MRI.createVirtualRegister(PtrRC);
110 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
112 BuildMI(MBB, InsertAddr, DL, TII->get(WebAssembly::CONST_I32), Zero)
114 MachineMemOperand *MMO = MF.getMachineMemOperand(
115 MachinePointerInfo(MF.getPSVManager().getExternalSymbolCallEntry(ES)),
116 MachineMemOperand::MOStore, 4, 4);
117 BuildMI(MBB, InsertStore, DL, TII->get(WebAssembly::STORE_I32))
118 .addImm(2) // p2align
119 .addExternalSymbol(SPSymbol)
125 MachineBasicBlock::iterator
126 WebAssemblyFrameLowering::eliminateCallFramePseudoInstr(
127 MachineFunction &MF, MachineBasicBlock &MBB,
128 MachineBasicBlock::iterator I) const {
129 assert(!I->getOperand(0).getImm() && (hasFP(MF) || hasBP(MF)) &&
130 "Call frame pseudos should only be used for dynamic stack adjustment");
131 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
132 if (I->getOpcode() == TII->getCallFrameDestroyOpcode() &&
133 needsSPWriteback(MF, MF.getFrameInfo())) {
134 DebugLoc DL = I->getDebugLoc();
135 writeSPToMemory(WebAssembly::SP32, MF, MBB, I, I, DL);
140 void WebAssemblyFrameLowering::emitPrologue(MachineFunction &MF,
141 MachineBasicBlock &MBB) const {
142 // TODO: Do ".setMIFlag(MachineInstr::FrameSetup)" on emitted instructions
143 auto &MFI = MF.getFrameInfo();
144 assert(MFI.getCalleeSavedInfo().empty() &&
145 "WebAssembly should not have callee-saved registers");
147 if (!needsSP(MF, MFI)) return;
148 uint64_t StackSize = MFI.getStackSize();
150 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
151 auto &MRI = MF.getRegInfo();
153 auto InsertPt = MBB.begin();
156 const TargetRegisterClass *PtrRC =
157 MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
158 unsigned Zero = MRI.createVirtualRegister(PtrRC);
159 unsigned SPReg = WebAssembly::SP32;
161 SPReg = MRI.createVirtualRegister(PtrRC);
162 const char *ES = "__stack_pointer";
163 auto *SPSymbol = MF.createExternalSymbolName(ES);
164 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), Zero)
166 MachineMemOperand *LoadMMO = MF.getMachineMemOperand(
167 MachinePointerInfo(MF.getPSVManager().getExternalSymbolCallEntry(ES)),
168 MachineMemOperand::MOLoad, 4, 4);
169 // Load the SP value.
170 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::LOAD_I32), SPReg)
171 .addImm(2) // p2align
172 .addExternalSymbol(SPSymbol)
173 .addReg(Zero) // addr
174 .addMemOperand(LoadMMO);
176 bool HasBP = hasBP(MF);
178 auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
179 unsigned BasePtr = MRI.createVirtualRegister(PtrRC);
180 FI->setBasePointerVreg(BasePtr);
181 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), BasePtr)
185 // Subtract the frame size
186 unsigned OffsetReg = MRI.createVirtualRegister(PtrRC);
187 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
189 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::SUB_I32),
195 unsigned BitmaskReg = MRI.createVirtualRegister(PtrRC);
196 unsigned Alignment = MFI.getMaxAlignment();
197 assert((1u << countTrailingZeros(Alignment)) == Alignment &&
198 "Alignment must be a power of 2");
199 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), BitmaskReg)
200 .addImm((int)~(Alignment - 1));
201 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::AND_I32),
203 .addReg(WebAssembly::SP32)
207 // Unlike most conventional targets (where FP points to the saved FP),
208 // FP points to the bottom of the fixed-size locals, so we can use positive
209 // offsets in load/store instructions.
210 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY),
212 .addReg(WebAssembly::SP32);
214 if (StackSize && needsSPWriteback(MF, MFI)) {
215 writeSPToMemory(WebAssembly::SP32, MF, MBB, InsertPt, InsertPt, DL);
219 void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF,
220 MachineBasicBlock &MBB) const {
221 auto &MFI = MF.getFrameInfo();
222 uint64_t StackSize = MFI.getStackSize();
223 if (!needsSP(MF, MFI) || !needsSPWriteback(MF, MFI)) return;
224 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
225 auto &MRI = MF.getRegInfo();
226 auto InsertPt = MBB.getFirstTerminator();
229 if (InsertPt != MBB.end())
230 DL = InsertPt->getDebugLoc();
232 // Restore the stack pointer. If we had fixed-size locals, add the offset
233 // subtracted in the prolog.
235 MachineBasicBlock::iterator InsertAddr = InsertPt;
237 auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
238 SPReg = FI->getBasePointerVreg();
239 } else if (StackSize) {
240 const TargetRegisterClass *PtrRC =
241 MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
242 unsigned OffsetReg = MRI.createVirtualRegister(PtrRC);
244 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
246 // In the epilog we don't need to write the result back to the SP32 physreg
247 // because it won't be used again. We can use a stackified register instead.
248 SPReg = MRI.createVirtualRegister(PtrRC);
249 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::ADD_I32), SPReg)
250 .addReg(hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32)
253 SPReg = hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32;
256 writeSPToMemory(SPReg, MF, MBB, InsertAddr, InsertPt, DL);